Uplink multiuser access may be implemented through different multiple access technologies, such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), CDMA and Space Division Multiple Access (SDMA). The multiuser CDMA communication technology is one of the very important uplink multiuser access communication technologies, and can achieve high access performance so as to be adopted by multiple wireless communication standards.
In an access process where CDMA is adopted, at first, each access terminal may spread, via a spreading process, a data symbol subjected to digital amplitude and/or phase modulation by virtue of a spread sequence with a certain length (for example, a spread sequence with a length L may refer to that the spread sequence is formed by L symbols or L elements, where the L symbols/L elements may be L digital symbols). The spreading process may refer to a process of multiplying each modulated data symbol by each symbol of the spread sequence to finally form a symbol sequence with the same length as the adopted spread sequence. In the spreading process, a modulated data symbol (for example, a constellation point symbol modulated by adopting Quadrature Amplitude Modulation (QAM)) may be multiplied by each symbol of the spread sequence, so that each modulated data symbol may finally be spread into a symbol sequence with the same length as the adopted spread sequence. For example, when the length of the spread sequence used is equal to L, each modulated symbol may be spread into L symbols, that is, each modulated data symbol is borne on a spread symbol sequence with the length L. Then, the spread symbol sequences of all the access terminals may be transmitted on the same time-frequency resource. Finally, a base station receives a combined signal formed by superimposing spread signals of all the access terminals, and useful information of each terminal is separated from the combined signal by virtue of a multiuser receiver technology.
A communication technology applying CDMA is usually classified into a spread spectrum communication category. This is because each modulated symbol of a terminal may be spread into L symbols, and if the transmission time of the spread L times of symbols is required to be equal to the transmission time of the modulated symbols before the spreading process, the required bandwidth has to inevitably be spread by L times. This is also why a spread sequence is usually called as a spread spectrum sequence.
If spread symbols of each terminal are transmitted through a multi-carrier technology (for example, Orthogonal Frequency Division Multiplexing (OFDM) and Filter Bank Multi-Carrier (FBMC)), a combination of the two technologies is called as a Multi-Carrier Code Division Multiple Access (MC-CDMA) technology.
In a CDMA technology, a spreading process of a transmitter side is relatively simple, since it is only required to multiply each modulated symbol, for example each symbol subjected to QAM, by each symbol of a spread sequence with a length L to obtain L spread symbols, and then transmit the spread symbols through a single-carrier or multi-carrier technology. Relatively, a receiving process at a base station is not so simple.
How to achieve high CDMA performance, or more directly, how can a base station accurately separate useful data information of each terminal from a combined signal is a key of a CDMA system. Two aspects are mainly involved in the receiving process: a spread sequence and a receiver. The selection of the spread sequence is a basis of the performance, and the design of the receiver is a guarantee of the performance.
In order to achieve high access performance, spread sequences adopted by different terminals are required to have good cross-correlation characteristics in the first place. If the spread sequences are directly transmitted in a wireless multipath channel by virtue of, for example, a single-carrier code division multiplexing technology, the sequences are also required to have good self-correlation characteristics to resist delay multipath spread of the sequences.
A multi-carrier code division multiplexing technology may resist multipath by virtue of a multi-carrier technology, so that only cross-correlation characteristic, favorable for multiuser information separation, of a spread sequence needs to be considered. This is the greatest difference between single-carrier code division multiplexing and multi-carrier code division multiplexing technologies in terms of sequence selection.
A good spread sequence is a basis of performance. Multiuser information separation is finally implemented on a base station side, and a base station may achieve different corresponding performances by adopting different multiuser receiving technologies. For achieving optimal multiuser data separation performance, the base station is required to adopt a multiuser receiver technology with high performance but high complexity, such as a Successive Interference Cancellation (SIC) receiver technology.
Just because of importance of a spread sequence, a main difference between different CDMA technologies lies in spread sequence selection. A Direct Sequence-Code Division Multiple Access (DS-CDMA) technology is a most common CDMA technology, and has been adopted by multiple wireless communication standards as an uplink multiuser access technology. The spread sequence adopted in the DS-CDMA technology is based on a simplest binary Pseudo-Noise (PN) real sequence. Due to simplicity of the sequence, PN-sequence-based DS-CDMA is one of most important multi-carrier code division multiplexing technologies. In the PN-sequence-based DS-CDMA technology, each modulated symbol may be spread by a binary PN real sequence before being transmitted out through a multi-carrier technology.
A binary PN real sequence may also be called as a binary PN sequence. A value of each symbol in the binary PN real sequence may usually be represented as 0 or 1, and may alternatively be represented as a bipolar sequence, for example, 0 is represented as +1 and 1 is represented as −1, or, 0 is represented as −1 and 1 is represented as +1.
The length of a spread sequence is also a key parameter of a CDMA technology. If spread sequences are longer, low cross-correlations between the spread sequences adopted by each terminal may be ensured more easily, and moreover, it would be easier to find more sequences with low cross-correlations, so that simultaneous access of more terminals may be supported. If the number of simultaneously accessing terminals is more than length of the spread sequence, it may be indicated that the multiuser access system is in an overloaded state. It is important to note that implementation of system overloading is one of shining key attributes of a CDMA technology in future wireless communications.
In order to provide a flexible system design and support simultaneous access of more users, spread sequences adopted by access terminals are usually not mutually orthogonal. From the point of a multiuser information theory, adopting a non-orthogonal multiple access manner in an uplink direction may achieve higher system capacity or edge throughput than an orthogonal multiple access manner. Since the spread sequences of different terminals are not mutually orthogonal, demodulation performance of each user may be worsened along with increase of the number of simultaneously accessing users under a normal circumstance. In case of system overloading, interference between multiple users may get more serious. At present, a relatively large application scenario of a CDMA technology is random access or resource competition access. Each access user transmits its own modulated symbol, spread by virtue of a spread sequence, in the same time-frequency resource, which means that the same time-frequency resource is used competitively.
In the communication technology, an important factor in CDMA constraining performance is that a user may spread all modulated symbols by virtue of one and the same spread sequence, that is, all the modulated symbols are spread by virtue of the same sequence. Such a manner brings convenience to implementation of a receiver with a SIC technology, and may simplify an implementation process of SIC. However, the solution of adopting a single spread sequence is unfavorable for access performance of non-orthogonal CDMA because interference between users may not be effectively randomized or averaged.
For the problem that interference between users may not be effectively randomized or averaged because one user spreads all modulated symbols by virtue of one and the same spread sequence in the communication technology, there is yet no effective technical solution.